Drilling mud, or drilling fluid, is provided during a drilling operation to provide various benefits including keeping a drill clean, removing cuttings from a bore hole, providing hydrostatic pressure to prevent fluids in a rock formation from entering the bore hole, and other benefits. The drilling mud comprises a fluid or filtrate, such as water and chemicals, and particulates, such as clay.
In a drilling operation, information regarding the formation is obtained by a wire line (WL) or by logging while drilling (LWD). The wire line data is obtained after the bore hole has been drilled, and the wire line equipment is inserted into the bore hole to obtain data. On the other hand, the LWD data is obtained simultaneously with the drilling operation.
Methods of logging include nuclear magnetic resonance (NMR) logging, neutron logging, and density logging. In NMR logging, a portion of a geological or rock formation is placed within magnetic fields to alter the magnetic state of the molecules in the formation. The characteristics of the formation are detected, including a T2 spectrum, which corresponds to the times for the molecules to return from an altered magnetic state to a relaxation state. The T2 spectrum is used to determine characteristics of the formation, such as porosity, or pore volume, and relative volumes of bound fluids and movable fluids in the geological formation.
In a LWD operation, various factors change measured characteristics of the formation including invasion of the filtrate into the formation and a slow-forming mud cake in the bore hole. These factors introduce uncertainties and variables in the calculations of density and hydrogen indexes in LWD operations.